All Thermalite® products, because of their closed cell structure, provide good resistance to the passage of moisture. Independent tests have indicated that Thermalite® blocks make water penetration extremely difficult.
Walls which contain an insulating material, such as Thermalite® blocks, can help reduce condensation. This is because the temperature drop across the wall is gradual.
Vapour resistivity
The vapour resistivity of Thermalite® is 60 MNs/gm.
Moisture penetration Both single leaf and cavity masonry walls of various configurations will resist moisture penetration if correctly designed, specified, and constructed. The proper standards of workmanship as defined in Annex A of BS 5628: Part 3 and BS 8000: Part 3 should be used.
Cavity walls Cavity walls with a minimum outer leaf thickness will perform acceptably in all Exposure Zones, (see table below). Thermalite® is commonly used as either the inner leaf to a brick outer leaf separated by a cavity, or as two separate leaves, finished externally with a moisture resistant or impervious cladding. A cavity wall with a rendered outer leaf of 100mm Shield will perform acceptably in zones of Very Severe exposure.
Generally, a 50mm cavity is satisfactory, but consideration should be given to wider cavities in conditions of Severe or Very Severe exposure.
Exposure zones
| Zone |
Description |
Quantity of wind driven rain litres/m²/spell* |
| 1 |
Sheltered |
Less than 33 |
| 2 |
Moderate |
33 to less than 56.5 |
| 3 |
Severe |
56.5 to less than 100 |
| 4 |
Very severe |
100 or more |
* Maximum wall spell index with reference to BS 8104
Single leaf walls Thermalite® is suitable for external solid walls, finished externally with a moisture resistant or impervious cladding (see below). To provide a weatherproof wall resistant to rain penetration, an appropriate external finish will be necessary. This is selected by reference to local exposure conditions as described in BRE Report 262 - Thermal insulation: avoiding risks.
Recommended thickness of single leaf masonry - for different finishes and exposure zones. For walls up to 12m high.
| Minimum block thickness (mm) |
Un-rendered |
Rendered as BS 5262 |
External insulation |
Impervious cladding* |
| 90 |
No |
Sheltered |
Severe |
Very severe |
| 190 |
No |
Moderate |
Severe |
Very severe |
| 215 |
Sheltered |
Severe |
Severe |
Very severe |
| 440 |
Moderate |
Severe |
Severe |
Very severe |
* Impervious cladding, as described in BS 5628 : Part 3, including slates, tile hanging, shingles, timber boarding or sheeting
Application of insulation Enhanced levels of insulation required by Building Regulations increase the risk of moisture penetration, if correct standards of specification, application and construction are not applied. The simplified table below indicates recommended wall constructions for insulated masonry walls.
Maximum recommended exposure zones for insulated masonry walls
| Insulation type |
Built-in full fill |
Injected full (not UF foam) |
Injected full (UF foam) |
Partial fill insulation |
Internal |
| Cavity width (mm) |
75 |
100 |
75 |
100 |
75 |
100 |
50*
residual |
50 clear |
50
full |
fill |
| Wall type impervious cladding (full height) |
4 |
4 |
4 |
4 |
4 |
4 |
4 |
4 |
4 |
4 |
| Rendered finish (full height) |
3 |
4 |
4 |
4 |
3 |
3 |
3 |
4 |
4 |
3 |
| Facing masonry (tooled flush joints) |
2 |
3 |
2 |
3 |
1 |
2 |
2 |
3 |
3 |
2 |
Refer to BRE Report 262 - Thermal insulation : avoiding risks for a full table of recommendations
Building Regulations for England & Wales, Scotland and Northern Ireland contain local requirements
*Partial fill cavities should maintain a minimum 50mm residual clear cavity width